The bottom casting for your Thermos grill is exactly the same as Char Broils aluminum gas grill, Home Depot & Lowe's sells a vaporizer bar & a lava rock grate for these grills. Like Mar52 said your choke'in off the air to burner Them ceramic's need to be on top the burner on a grate.
Look on the left & right side in your grill botom's side', those piramid shape castings are what support a flavorizer bar. Now look in the bottoms font & back you will see 2 cast in tabs. They can be seen in the photos.
Those are made to support a rock grate.
As stated use either or... Not both.
I have the same grill made by Char Broil. I made a grate out of hardware cloth to set inplace of the rock grate. Now I put charcoal & wood on it. I use the burner to ignite the coals. Coals & wood make a way better tasting finished meal. cooking with gas on a grill is just like Broiling in a Oven.
I dont worry about the ash just brush it off, Burners are cheap.
BTW Lowe's & Home Depot sells replacement burners too
I think This mod is shown in more than one of my over 500 videos...
Look here at about 2:55 into the clip;
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